Secondary remanent magnetization carried by magnetite inclusions in silicates: a comparative study of unremagnetized and remagnetized granites
Secondary remanent magnetization carried by magnetite inclusions in silicates: a comparative study of unremagnetized and remagnetized granites
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硅酸盐中磁铁矿包裹体携带的二次剩磁：未再磁化和再磁化花岗岩的比较研究
DOI：
10.1016/s0012-821x(00)00169-2
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发表时间：
2000
影响因子：
5.3
通讯作者：
T. Matsuda
中科院分区：
文献类型：
--
作者：
Y. Otofuji;K. Uno;Takahiro Higashi;Tomomichi Ichikawa;Tsuyoshi Ueno;T. Mishima;T. Matsuda
利用古地磁学、岩石磁学、光学显微镜和扫描电子显微镜（SEM）研究了日本东北部重磁化白垩纪花岗岩中的磁性载体，并与未磁化花岗岩进行了对比。再磁化岩石的自然剩磁强度（0.3- 1.7A/m）强，为北西向，倾向中等（NW剩磁），而未磁化岩石的自然剩磁强度（<0.5A/m）弱，为西向，倾向浅（W剩磁）。虽然热退磁表明这两种NRM都是由磁铁矿携带的，但重磁化岩石相对于交变场退磁（20 mT<中值破坏场）比未磁化岩石（<10 mT）显示出更高的磁化率。根据修正的Lowrie-Fuller试验和磁滞参数，未磁化岩石中含有多畴颗粒，而重磁化岩石中的磁性载体则是多畴颗粒和单畴颗粒的混合体。光学检查表明，尽管未磁化和重磁化岩石中常见的磁性颗粒是钛磁铁矿颗粒，其钛铁矿片层直径大于30 μm，但重磁化岩石的特征是大量小于10 μm的纯磁铁矿颗粒。扫描电镜研究表明，这些细小的纯磁铁矿颗粒以分散的颗粒形式出现在阳起石化部分，其中Fe/Mg摩尔比为初级角闪石的三分之一。这些观察结果表明，NW剩磁在重磁化岩石驻留在细纯磁铁矿包裹体二次形成从释放的铁离子在角闪石的阳起石蚀变。NW剩磁是在62 ~ 15 Ma的某个时间岩石蚀变过程中获得的化学剩磁。在未磁化和再磁化岩石中观察到的原始形成的粗粒钛磁铁矿携带白垩纪获得的热还原磁化的W剩磁。
Magnetic carriers in remagnetized Cretaceous granitic rocks of northeast Japan were studied using paleomagnetism, rock magnetism, optical microscopy and scanning electron microscopy (SEM) by comparison with unremagnetized granitic rocks. The natural remanent magnetization (NRM) of the remagnetized rocks is strong (0.3–1.7 A/m) and shows a northwesterly direction with moderate inclination (NW remanence), whereas the unremagnetized rocks preserve weak NRM (<0.5 A/m) with westerly and shallow direction (W remanence). Although thermal demagnetization shows that both NRMs are carried by magnetite, the remagnetized rocks reveal a higher coercivity with respect to alternating field demagnetization (20 mT<median destructive field) than the unremagnetized rocks (<10 mT). On the basis of the modified Lowrie–Fuller test and hysteresis parameters, the unremagnetized rocks carry multi-domain grains whereas the magnetic carrier of the remagnetized rocks is described as a mixture of multi- and single-domain grains. Optical examination reveals that although common magnetic particles in the unremagnetized and remagnetized rocks are titanomagnetite grains with ilmenite lamellae larger than 30 μm, numerous pure-magnetite grains finer than 10 μm are characteristic of the remagnetized ones. Studies by SEM equipped with an energy-dispersive analytical system show that these fine pure-magnetite grains occur as discrete particles within actinolitized parts where the Fe/Mg mole ratio is one third of primary hornblende. These observations suggest that the NW remanence in the remagnetized rocks resides in the fine pure-magnetite inclusions secondarily formed from released Fe ions during the alteration of hornblende to actinolite. The NW remanence is a chemical remanent magnetization which was acquired during the alteration of rocks at some time between 62 and 15 Ma. The originally formed coarse-grained titanomagnetite observed in both unremagnetized and remagnetized rocks carry the W remanence of a thermoremanent magnetization acquired in the Cretaceous time.